1. Field of the Invention
The present invention relates to a method and related communication device for a trigger mechanism of buffer status report (BSR) and scheduling request (SR) in a wireless communication system, and more particularly, to a method and related communication device for using a threshold to decide when to trigger a BSR-SR.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
In the 3GPP associated specifications, logical channels are defined as service access points between a Media Access Control (MAC) layer and a Radio Link Control (RLC) layer. The MAC provides data transfer services on logical channels. Each logical channel type is defined by the type of information to be transferred. A Radio Resource Control (RRC) layer can control the scheduling of uplink data by giving each logical channel a priority.
According to the current 3GPP MAC specification, a buffer status reporting procedure is used to provide information about the size of data in uplink (UL) buffers of a UE for a serving eNB. A buffer status report (BSR) is triggered when UL data belonging to a logical channel with higher priority than those for which data already existed in the UE transmission buffer arrives at a UE transmission buffer. In addition, a scheduling request (SR) is used to request UL resources. The SR is triggered when the UE does not have a UL resource allocated for the current transmission time interval (TTI), which implies that a dedicated SR (D-SR) is transmitted on the physical uplink control channel (PUCCH) if the UL resource is allocated to the UE, or alternatively a random access SR (RA-SR) is transmitted on the random access channel (RACH). The pending SR is cancelled when new resources are available on UL-SCH, which is granted by eNB through dynamically scheduling.
In order to utilize shared channel (SCH) resources, a dynamic scheduling (DS) function is used in MAC. When sharing resources between UEs, MAC in eNB dynamically allocates physical layer resources for the DL-SCH and UL-SCH depends on the traffic volume, the QoS requirements of each UE and associated radio bearers. On the other hand, a semi-persistent scheduling (SPS) is introduced in LTE system and is also used in MAC for serving upper layer applications which generates semi-static size data periodically, e.g. VoIP services. SPS is more efficient than DS for VoIP data transmission.
According to the current UE MAC specification, an SR is triggered without considering the already allocated UL grant requested through DS or assigned by SPS. Only the current TTI is considered when making the decision of triggering the SR. This UE behavior results in several issues described as follows.
The first issue is described as follows. Please refer to FIG. 1, which is a timing diagram illustrating a relationship between an allocated UL grant and an SR in dynamic scheduling according to the prior art. In FIG. 1, there is a time gap Tg between a physical downlink control channel (PDCCH) UL grant and an actual UL transmission on a physical uplink shared channel (PUSCH), typically around 4 ms in E-UTRAN. In other words, when a UL grant is received in subframe n, the actual UL transmission takes place in subframe (n+4). After receiving a PDCCH UL grant, the UE decodes and processes the received information, which takes a processing time Tp, normally less than 2 ms. Therefore, there is a window with a length (Tg-Tp) in which the UE acknowledges the upcoming allocated UL grant 1 but does not have a UL resource allocation for several TTIs before the upcoming UL grant 1.
During this window, if new UL data arrives at a transmission buffer and the new UL data belongs to a logical channel with higher priority than those for which data already existed in the transmission buffer, a BSR and the associated SR, abbreviated to BSR-SR, is triggered. If the total available data (new arriving data plus the existed data) in the transmission buffer could be accommodated in the upcoming allocated UL grant 1, the transmission buffer would be empty after the allocated UL grant 1, and the latter assigned UL grant 2 is therefore wasted. In this situation, BSR-SR triggering is unnecessary.
When SPS resources are configured, the second issue happens and is described as follows. Please refer to FIG. 2, which is a timing diagram illustrating a relationship between an allocated SPS UL resource and an SR according to the prior art. As shown in FIG. 2, SPS data, such as VoIP data, enters the transmission buffer and an SR is generated. An assigned UL grant may arrive before or after an SPS resource, as a UL grant 1 or a UL grant 2 shown in FIG. 2. If the UL grant comes before a certain SPS resource, the SPS data is transmitted in the UL grant 1 and the latter SPS resource is left empty. On the other hand, if the UL grant comes after the certain SPS resource, the SPS data is transmitted in the latter SPS resource and the UL grant 2 is left empty. The SR in the above situation is called a “premature SR”.
Premature SR also results from the fact that there is no guarantee that SPS data packets delivery is synchronized with periodic SPS resources. Generally, periodicity of SPS data delivery from a higher layer and periodicity of SPS resources are identical. Note that, when SPS data arrives at the transmission buffer, the UE needs a processing time to process the SPS data before it is actually transmitted. If the SPS data is ready for transmission at the TTI of SPS resource, i.e. the two processes are “synchronized”. If the SPS data does not catch the SPS resource, which is called “unsynchronized”, the UE considers that it does not have an SPS UL resource allocated for the current TTI and a premature SR is triggered.
In accordance with the first issue and the second issue, it is known that wasting an allocated UL grant results in unnecessary BSR-SR triggering and inefficient use of UL resources. Moreover, the problem deteriorates in the presence of SPS, which is designed for time-critical applications.
The third issue is described as follows. Note that when SPS resources are configured, data belonging to a logical channel except SPS logical channels is called lower priority data. According to the current specification, when lower priority data arrives at the transmission buffer, aBSR is not triggered when SPS data available for transmission is never emptied and simply sits in the transmission buffer. As a result, the potential starvation for transmission of the lower priority data may happen.
Besides, in the current MAC specification, a pending SR shall be cancelled until UL-SCH resources are granted for a new transmission. It is not clear whether these UL-SCH resources for new transmission include periodic new SPS transmissions without any PDCCH assignment. If periodic SPS transmissions would cancel the pending SR, the short-lived SR may be cancelled prematurely, and even worse, cancelled periodically. Please refer to FIG. 3, which is a timing diagram illustrating a relationship between SPS resources and lower priority data according to the prior art. As shown in FIG. 3, starvation for transmission of lower priority data happens when SPS data available for transmission is never emptied.
The fourth issue is described as follows. There are three types of BSRs for different triggering events, a regular BSR, a periodic BSR and a padding BSR. The regular BSR is triggered when UL data arrives at the UE transmission buffer and the UL data belongs to a logical channel with higher priority than those for which data already existed in the UE transmission buffer, or is triggered when a serving cell change occurs. The periodic BSR is triggered when a periodic BSR timer expires. The padding BSR is triggered when UL resources are allocated and an amount of padding bits is equal to or greater than the size of the BSR MAC control element.
Besides, there are three types of BSR format, long, short, and truncated BSR. Please refer to FIG. 4, which is a table of triggering events and corresponding BSR formats according to the prior art. Long BSR is used by regular BSR and periodic BSR if there are more than two logical channel groups (LCGs) having buffered data, and is used by padding BSR if the amount of padding bits is large enough. Short BSR is used by regular BSR and periodic BSR if there is only one LCG having buffered data, and is used by padding BSR if there is only one LCG having buffered data and the amount of padding bits is not large enough for long BSR. The truncated BSR is used by padding BSR when there are more than one LCG having buffered data and the amount of padding bits is not large enough for long BSR.
Before SPS is introduced, there is no obstacle for the eNB to know the real buffer status of the UE through the current BSR mechanism. However, when SPS is configured, the current BSR mechanism becomes sub-optimal. For padding BSR in the current specification, when the number of non-empty LCG is greater than 1, the UE reports a truncated BSR of the LCG with the highest priority logical channel. When SPS is configured, the highest priority logical channel is usually SPS logical channel and therefore the truncated BSR would always report LCG of SPS logical channel. However, since the SPS resources are already allocated, most of time, the eNB does not really need that information.
The fifth issue is described as follows. Please refer to FIG. 5, which is a timing diagram of a multiple-SPS-pattern scheme for TDD (Time Division Duplex) mode. The multiple-SPS-pattern scheme is designed to deal with frequent collision between initial transmissions and retransmissions. Two different intervals, T1 and T2, interchange with each other continuously and the sum of two intervals equals two times of the SPS period.
Therefore, even though the average SPS UL resource is still one per SPS period, the separation between adjacent SPS resources is not identical anymore. Another way to understand is there are two patterns sharing the same period but being initiated at different times. SPS resource allocation in TDD mode meets the same problems, e.g. premature SR and starvation for transmission of lower priority data, as it in FDD (Frequency Division Duplex) mode.